Going from Boston to New York but not sure exactly when I'll be going, within a timeframe of an hour or two, as I'm coming from another location and won't know till I arrive.

What is the protocol if I make a reservation for 7 p.m. but I decide that I want to go at 8 p.m. or 6 p.m.? 

MegaBus, Greyhound, Bolt, PeterPan etc.